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Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
The effect on sitting posture of a desk with a 10° inclination for reading and writing a
a
a
M. DE WALL , M. P. J. M. VAN RIEL , C. J. SNIJDERS & J. P. VAN WINGERDEN
b
a
Department of Biomedical Physics and Technology , Faculty of Medicine, Erasmus University Rotterdam , PO Box 1738, Rotterdam, 3000 DR, The Netherlands b
Department of Anatomy , Faculty of Medicine, Erasmus University Rotterdam , PO Box 1738, Rotterdam, 3000 DR, The Netherlands Published online: 31 May 2007.
To cite this article: M. DE WALL , M. P. J. M. VAN RIEL , C. J. SNIJDERS & J. P. VAN WINGERDEN (1991) The effect on sitting posture of a desk with a 10° inclination for reading and writing, Ergonomics, 34:5, 575-584, DOI: 10.1080/00140139108967338 To link to this article: http://dx.doi.org/10.1080/00140139108967338
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
The effect on sitting posture of a desk with a 10"inclination for reading and writing M.DE WALL,M. P.J. M. VAN RIELand C. J. SNLTDERS Department of Biomedical Physics and Technology, Faculty of Medicine, Erasmus University Rotterdam, PO Box 1 738, 3000 DR, Rotterdam, The Netherlands
Downloaded by [University of Arizona] at 00:04 08 September 2015
and J. P.
VAN
W~GERDEN
Department of Anatomy, Faculty of Medicine, Erasmus University Rotterdam, PO Box 1738, 3000 DR, Rotterdam, The Netherlands Keywords: Posture; Cervical spine; Head; Inclined desk; Workplace design; Biomechanics. A pilot study was camed out on 10 subjects to examine the effect that a desk with a 10" inclination had on their sitting posture while reading and writing. A
continuous recording of the subjects head and trunk positions was made. Each subject was studied during two periods of 1-5 h on different days. The subject worked at a flat desk on the first day and at a desk with a 10" inclination on the second. On average, the position of the head in the sagittal plane was found to be 6" more erect and the position of the trunk 7" more erect when working at a desk with a 10" inclination than when working at a flat desk. The maximal decrease in load observed on the cervical spine was 35% and on the thoracic spine 95%. The angle between the head and the trunk did not change significantly when using an inclined desk. Differences in posture in the frontal plane were not observed in this study.
1. Introduction Several authors have studied the relationship between bad working posture and physical complaints (Onishi et al. 1976, Meada 1977, Hiinting 1980, Kvarnstrijm 1983, Hagberg 1987, Van Straten 1987). In some of these studies, prolonged static muscle contraction appears to be related to head, neck and shoulder pain. The degree of fonvard bending of the head also appears to be related to the incidence of neck complaints. The shape of the chair is important for a good sitting posture. Schiildt et al. (1 986) found that an adjustable ofice chair with good lumbar support, good armrests and a 10- 15" backward inclination reduces neck and shoulder muscle activity significantly. Some office activities such as reading and writing, however, draw the head and trunk in the direction of the document lying on the desk. A stooped posture will therefore be adopted in spite of the chair (figure 1). It is clear from figure 2 that with the head bent forward the force (F)which the neck muscles have to produce to keep it in a balanced position must increase. During forward bending of the head, the horizontal distance between the centre of gravity (G) of the head and the axis of articulation of the head (A) in the atlanto-occipital joint increases. N in this figure is the reaction force in the atlanto-occipital joint. Because of the static nature of this increased force in the neck muscles during desk work, it may caused diminished blood flow in the muscles and tissue damage. This can give rise to pain in the neck or shoulders. Pain in turn may lead to reflex muscle contraction of spasm, thus completing the vicious circle. 0014-013919 1 S 3-00 O 199 1 Taylor & Francis Ltd.
Downloaded by [University of Arizona] at 00:04 08 September 2015
M.de Wall et al.
Figure 1 . Head and trunk drawn in the direction of the document lying on the desk, from Snijders ( 1 984).
Figure 2. Increase in neckmuscle force in fonvard bent position, from Snijders ( 1 984).
One can try to break this circle in several ways. For example, by using analgesics or drugs to combat muscle spasms or by increasing blood flow by means of physiotherapy. It is preferable, however, to avoid the vicious circle. This can be done by staying in one position for only a short time. Another way is to lower the force the neck muscles must produce to keep the head in a balanced position. The position in which one leans over the back of the chair is particularly effective in lowering tension in the neck and back (Anderson and t)rlengren 1974). Because of the difficulty of doing desk work in this position, other less strenuous sitting positions have been searched for. A logical alternative would seem to be to use an inclined working surface, for it is thought that this results in a more upright position. Sloping desks were still to be found in offices and schools as recently as 20 years
Downloaded by [University of Arizona] at 00:04 08 September 2015
The eflect on posture of an inclined desk
577
ago. Nowadays only draughtsmen use inclined working surfaces. The reason that such surfaces have been abandoned probably lies in the fact that papers and pens have a constant tendency to slide down them.To avoid this inconvenience, a desk with a 10" inclination has now been designed at Erasmus University in Rotterdam. The question which then presents itself is whether an inclination of 10" is sufficient to cause a more erect sitting posture during reading and writing. A new method has recently been developed for the continuous recording of posture (Snijders et al. 1987). This method allows us to compare sitting postures at a flat desk and at an inclined desk. The aim of this study is to make a contribution to ergonomic knowledge of desk work, with special reference to reading and writing. To achieve this we ask the following questions. Does the preferred posture change when using an inclined desk? If so, does the person bend forward less or more than when working at a flat desk? Does the range of postures which a person adopts during work change when using an inclined desk? In other words, does the person shift position more often? 2. Choice of subjects The aim of this study is to gather information which can lead to an improvement in workplace design. The subjects must therefore be chosen from a population of potential users of the type of workplace in question. Examples of such workplaces are studies, schools, examination rooms, offices and conference rooms. Students and staff members at the university were an obvious choice of subjects. The specific conditions which the subjects had to fulfil were:
(1) aged between 20 and 60 years; (2) accustomed to desk work; (3) height between 1- 60m and 1~ 9 m; 5 (4) .motivated to cooperate; (5) good eyesight, or adequate presbyopic correction.
The measurements were performed on 10 subjects, nine men and one woman. One subject had an optical correction for presbyopia. 3. Materials and methods To record the position of both the head and the trunk,two inclinometers were put on both: one to record the movements in the sagittal plane, the other for movements in the frontal plane. The location of the inclinometers had to be such that they gave a true indication of the posture of that part of the body. Because of its rigid shape, it did not matter where the inclinometers were placed on the head. It was decided to attach them on top of the head by means of a kind of cycling helmet (weight 170g, inclusive of the two inclinometers). To measure the position of the trunk, the inclinometers were placed on the back on the level of the first and second thoracic vertebrae. This is the least flexible part of the spine. It also has the advantage that the meters are most easily attached here because of the subject's clothes and that there is no danger of their moving as a result of contact with the back of the chair. The inclinometers were placed in the medial line so that shoulder movements caused no disturbances in the recordings. The two inclinometers placed on the head were finnly attached to the helmet, which was adjustable for each subject and was stable on the head once it had been adjusted. The
Downloaded by [University of Arizona] at 00:04 08 September 2015
578
M.de WaN et al.
two inclinometers on the back of the trunk were attached with FixomuU Stretcha tape after the fat had been removed from the skm with alcohol. In previous research aimed at improving posture by using an inclined desk, the wide spread in the results was the main reason that it was not possible to amve at any conclusions (Smeets et al. 1982). Because of this, we chose to do a pilot study with a small number of subjects (n = 10) in which the only experimental variable was the use of a desk with a 10" inclination or a flat desk. The recordings were camed out in a laboratory. Each of the subjects sat on the same chair and at the same desk during the recordings. The chair had a seat height of 47 cm, a slightly inclined back and armrests (height: 68 crn). The desk was 76 cm high (desksurface 50 x 4 1 cm). The lightingwas fixed and not adjustable. The recordings were performed twice, one with the inclined desk and once with the flat desk. The two recordings were made on different days but at around the same time of day. The subjects used the same reading matter on both days. The type and size of the letters was therefore the same during the two recordings. The subjects brought their own reading matter in order to ensure concentration and cooperation. Because of the concentration factor, the recording time was also kept short: 2 x 45 min with a 15 min coffee break. After the first 45 rnin of recording their reading posture, the subjects were asked to write for Smin in order to get an impression of the influence of an inclined desk on writing posture. Before, between and after the two sessions, a gauge was recorded on tape. This involved recording the 'zero position* in addition to some extreme body positions. The zero position is the position in which the subject stands erect with an upright head, and with the left thumb in the right fist. A mark on the wall at 5 m distance ensures that the subjects looks forward at an angle of 15" below the horizontal. Besides the four inclinometers, use was made of an instrumentation recorder (Racall Store 14). For a detailed description of the inclinometer, see'snijders et a L (1987). An inclinometer measures its angle in relation to the direction of gravity. The aforementioned zero position is given the value 0".A forward bending posture gets a positive angle, and a backward bending, a negative angle. This method has the advantage that if the inclinometer was attached at a slightly different place during the second recording, then it would not influence the results. It also reduced individual differences. The analogue signal (figure 3) was digitized and fed into a computer (PDP 11/73). During this procedure, the tape was accelerated to 32 times the recording speed. In order to record spinal movements with a frequency up to 3 Hz, a filter frequency up to 96 Hz and a sample frequency of 480 Hz were used. With the aid of a number of computer programs, each signal was converted into a histogram in which, using 2" intervals, the time that the person stayed in a posture is given as a percentage (Snijders et al. 1987). By computing the median of the aforementioned percentages an indication can be obtained of the subject's preferred posture. The range of posture lies between the position of the 5th and 95th percentiles in the histograms (so posture is in this range 90% of the time). 4. Statistics Whether the use of an inclined desk as opposed to a flat desk result in a change in preferred posture can be determined by comparing the aforementioned medians of the two recordings of the subjects. A difference in sitting behaviour can be identified by comparing the ranges of posture in these recordings.
579
Downloaded by [University of Arizona] at 00:04 08 September 2015
The efSect on posture of an inclined desk
looJ scc o
. . . . . . . . . . . . . . . . . . . . . . . . . r
too
.
r
is0
1
.
1
.
,
1
200
.
1
.
is0
Figure 3. The analogue signal of two inclinometers.
Wilcoxon's test was used to compare quantitative differences between paired observations against the zero hypothesis, because the elements of the random test come from a symmetrically distributed population with a mean of zero. 5. Results An example of the calculated histograms for one person are presented in figure 4. This figure shows the histograms of the sagittal flexion of the head, trunk and the angle between head and trunk during one session with the flat desk. The histograms of most of the subjects show a shift to a more upright posture when using the inclined desk. An example is given in figure 5, in which working at the inclined desk is indicated by the shaded histogram and working at the flat desk by the black histogram. The medians of these histograms were taken to be the preferred postures. In the reading test, the mean value of the preferred postures of the trunks of the 10 subjects
-
Downloaded by [University of Arizona] at 00:04 08 September 2015
M.de Wall et al.
Figure 4. Result of the canversion of the analogue signal into histograms. Zero degrees corresponds with a posture in the 'zero position'.
in the sagittal plane was 7"more upright. One subject had a 28"more upright position of the trunk. For the head, the change in preferred posture was an elevational of 6" to a more upright position (with a maximum of 16' for two subjects). Both differences proved to be significant (pt0.05)(table 1). The angle between the head and trunk in the sagittal plane did not change significantly. Similarly, the inclined desk did not result in a difference in the preferred posture in the frontal plane (table 1).
.
Downloaded by [University of Arizona] at 00:04 08 September 2015
The eflect on posture ofan inclined desk
0 -20-10
0 10 20 X) 40 w l e between head ud mrk
50
60
70
Iawrnl
Figure 5. Examples of histograms of one subject when reading with an without an inclined desk.
As we11 as decreasing during the reading test, the angEe of the trunk in the sagittal plane also decreased by 9" when the subjects wrote at the inclined desk (table 3). However, a decrease in the sagittal angle of the head during writing could not be demonstrated. The range of trunk postures in the frontal plane decreased by 4" (pt0.05)when
M. de Wall et al.
582
Table 1. Mean of the preferred postures of the 10 subjects in degrees in relation to the 'zeroposition' during the reading test. Inclined desk
Posture
32 23 9 4 2 3
Head in sagittal plane
Trunk in sagittaI plane Head-trunk in sagittal plane Head in frontal plane Trunk in frontal plane Head-trunk in frontal plane
Flat desk
38 ( 2 7 . 1 ) 30 ( i 7 . 5 ) 8 (k6.0) 6 (26.7) 1 (k2.8) 5 (k5.3)
(k6-9) (k4.5) (rt 7.0) (*3-9) (~2.2) ( k 5.1)
Difference - 6 (k5-7)* -7 (k8.6). 1 (k7.4)
-2 (k3.9) 1 (k2.7) -2 (-3.0)
Downloaded by [University of Arizona] at 00:04 08 September 2015
*pcO.05; Wilcoxon's test.
Table 2. Mean of the ranges of posture (the difference between the 5th and the 95th percentile) of the 10 subjects in degrees during the reading test. Inclined -
-
Flat desk
desk
Posture - -
Head in sagittal plane Trunk in sagittal plane Head-trunk in sagittal plane Head in frontal plane Trunk in frontal plane Head-trunk in frontal plane
-
24 27 30 24 :
Difference -
(rt 8.5)
24 ( k 8.5) 20 ( * 4.1) 26 ( * 7-5) 27 (k 10.0) 1 6 ( * 5-9) 27 (k 12-9)
( 2 10-4)
(k10.6) (*10.6)
12 ( 2 4-0) 23 ( k .. 8.8)
0 ( * 7-4) 7 (i17-1) 4 ( A 13.3) -3 (+. 4-9) -4(* 2.3). -4 (-c 8-2)
*pt0.05; Wilcoxon's test.
reading at an inclined desk (table 2). This could not be demonstrated during the writing test, perhaps because of its short duration (5 min). 6. Discussion This method has been a workable one for recording subjects' posture. Technical defects did not occur and the test had good reproducibility. The value of the 'zere position' was measured three times at each session. The three 'zero positions' of each '
Table 3. Mean of the preferred postures of the 10 subjects in degrees in relation to the 'zem position' during the writing test. Posture
Head in sagittal plane
Trunk in sagittal plane Head-trunk in sagittal plane Head in frontal plane Trunk in frontal plane Head-trunk in frontal plane
*p
Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
The effect on sitting posture of a desk with a 10° inclination for reading and writing a
a
a
M. DE WALL , M. P. J. M. VAN RIEL , C. J. SNIJDERS & J. P. VAN WINGERDEN
b
a
Department of Biomedical Physics and Technology , Faculty of Medicine, Erasmus University Rotterdam , PO Box 1738, Rotterdam, 3000 DR, The Netherlands b
Department of Anatomy , Faculty of Medicine, Erasmus University Rotterdam , PO Box 1738, Rotterdam, 3000 DR, The Netherlands Published online: 31 May 2007.
To cite this article: M. DE WALL , M. P. J. M. VAN RIEL , C. J. SNIJDERS & J. P. VAN WINGERDEN (1991) The effect on sitting posture of a desk with a 10° inclination for reading and writing, Ergonomics, 34:5, 575-584, DOI: 10.1080/00140139108967338 To link to this article: http://dx.doi.org/10.1080/00140139108967338
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
The effect on sitting posture of a desk with a 10"inclination for reading and writing M.DE WALL,M. P.J. M. VAN RIELand C. J. SNLTDERS Department of Biomedical Physics and Technology, Faculty of Medicine, Erasmus University Rotterdam, PO Box 1 738, 3000 DR, Rotterdam, The Netherlands
Downloaded by [University of Arizona] at 00:04 08 September 2015
and J. P.
VAN
W~GERDEN
Department of Anatomy, Faculty of Medicine, Erasmus University Rotterdam, PO Box 1738, 3000 DR, Rotterdam, The Netherlands Keywords: Posture; Cervical spine; Head; Inclined desk; Workplace design; Biomechanics. A pilot study was camed out on 10 subjects to examine the effect that a desk with a 10" inclination had on their sitting posture while reading and writing. A
continuous recording of the subjects head and trunk positions was made. Each subject was studied during two periods of 1-5 h on different days. The subject worked at a flat desk on the first day and at a desk with a 10" inclination on the second. On average, the position of the head in the sagittal plane was found to be 6" more erect and the position of the trunk 7" more erect when working at a desk with a 10" inclination than when working at a flat desk. The maximal decrease in load observed on the cervical spine was 35% and on the thoracic spine 95%. The angle between the head and the trunk did not change significantly when using an inclined desk. Differences in posture in the frontal plane were not observed in this study.
1. Introduction Several authors have studied the relationship between bad working posture and physical complaints (Onishi et al. 1976, Meada 1977, Hiinting 1980, Kvarnstrijm 1983, Hagberg 1987, Van Straten 1987). In some of these studies, prolonged static muscle contraction appears to be related to head, neck and shoulder pain. The degree of fonvard bending of the head also appears to be related to the incidence of neck complaints. The shape of the chair is important for a good sitting posture. Schiildt et al. (1 986) found that an adjustable ofice chair with good lumbar support, good armrests and a 10- 15" backward inclination reduces neck and shoulder muscle activity significantly. Some office activities such as reading and writing, however, draw the head and trunk in the direction of the document lying on the desk. A stooped posture will therefore be adopted in spite of the chair (figure 1). It is clear from figure 2 that with the head bent forward the force (F)which the neck muscles have to produce to keep it in a balanced position must increase. During forward bending of the head, the horizontal distance between the centre of gravity (G) of the head and the axis of articulation of the head (A) in the atlanto-occipital joint increases. N in this figure is the reaction force in the atlanto-occipital joint. Because of the static nature of this increased force in the neck muscles during desk work, it may caused diminished blood flow in the muscles and tissue damage. This can give rise to pain in the neck or shoulders. Pain in turn may lead to reflex muscle contraction of spasm, thus completing the vicious circle. 0014-013919 1 S 3-00 O 199 1 Taylor & Francis Ltd.
Downloaded by [University of Arizona] at 00:04 08 September 2015
M.de Wall et al.
Figure 1 . Head and trunk drawn in the direction of the document lying on the desk, from Snijders ( 1 984).
Figure 2. Increase in neckmuscle force in fonvard bent position, from Snijders ( 1 984).
One can try to break this circle in several ways. For example, by using analgesics or drugs to combat muscle spasms or by increasing blood flow by means of physiotherapy. It is preferable, however, to avoid the vicious circle. This can be done by staying in one position for only a short time. Another way is to lower the force the neck muscles must produce to keep the head in a balanced position. The position in which one leans over the back of the chair is particularly effective in lowering tension in the neck and back (Anderson and t)rlengren 1974). Because of the difficulty of doing desk work in this position, other less strenuous sitting positions have been searched for. A logical alternative would seem to be to use an inclined working surface, for it is thought that this results in a more upright position. Sloping desks were still to be found in offices and schools as recently as 20 years
Downloaded by [University of Arizona] at 00:04 08 September 2015
The eflect on posture of an inclined desk
577
ago. Nowadays only draughtsmen use inclined working surfaces. The reason that such surfaces have been abandoned probably lies in the fact that papers and pens have a constant tendency to slide down them.To avoid this inconvenience, a desk with a 10" inclination has now been designed at Erasmus University in Rotterdam. The question which then presents itself is whether an inclination of 10" is sufficient to cause a more erect sitting posture during reading and writing. A new method has recently been developed for the continuous recording of posture (Snijders et al. 1987). This method allows us to compare sitting postures at a flat desk and at an inclined desk. The aim of this study is to make a contribution to ergonomic knowledge of desk work, with special reference to reading and writing. To achieve this we ask the following questions. Does the preferred posture change when using an inclined desk? If so, does the person bend forward less or more than when working at a flat desk? Does the range of postures which a person adopts during work change when using an inclined desk? In other words, does the person shift position more often? 2. Choice of subjects The aim of this study is to gather information which can lead to an improvement in workplace design. The subjects must therefore be chosen from a population of potential users of the type of workplace in question. Examples of such workplaces are studies, schools, examination rooms, offices and conference rooms. Students and staff members at the university were an obvious choice of subjects. The specific conditions which the subjects had to fulfil were:
(1) aged between 20 and 60 years; (2) accustomed to desk work; (3) height between 1- 60m and 1~ 9 m; 5 (4) .motivated to cooperate; (5) good eyesight, or adequate presbyopic correction.
The measurements were performed on 10 subjects, nine men and one woman. One subject had an optical correction for presbyopia. 3. Materials and methods To record the position of both the head and the trunk,two inclinometers were put on both: one to record the movements in the sagittal plane, the other for movements in the frontal plane. The location of the inclinometers had to be such that they gave a true indication of the posture of that part of the body. Because of its rigid shape, it did not matter where the inclinometers were placed on the head. It was decided to attach them on top of the head by means of a kind of cycling helmet (weight 170g, inclusive of the two inclinometers). To measure the position of the trunk, the inclinometers were placed on the back on the level of the first and second thoracic vertebrae. This is the least flexible part of the spine. It also has the advantage that the meters are most easily attached here because of the subject's clothes and that there is no danger of their moving as a result of contact with the back of the chair. The inclinometers were placed in the medial line so that shoulder movements caused no disturbances in the recordings. The two inclinometers placed on the head were finnly attached to the helmet, which was adjustable for each subject and was stable on the head once it had been adjusted. The
Downloaded by [University of Arizona] at 00:04 08 September 2015
578
M.de WaN et al.
two inclinometers on the back of the trunk were attached with FixomuU Stretcha tape after the fat had been removed from the skm with alcohol. In previous research aimed at improving posture by using an inclined desk, the wide spread in the results was the main reason that it was not possible to amve at any conclusions (Smeets et al. 1982). Because of this, we chose to do a pilot study with a small number of subjects (n = 10) in which the only experimental variable was the use of a desk with a 10" inclination or a flat desk. The recordings were camed out in a laboratory. Each of the subjects sat on the same chair and at the same desk during the recordings. The chair had a seat height of 47 cm, a slightly inclined back and armrests (height: 68 crn). The desk was 76 cm high (desksurface 50 x 4 1 cm). The lightingwas fixed and not adjustable. The recordings were performed twice, one with the inclined desk and once with the flat desk. The two recordings were made on different days but at around the same time of day. The subjects used the same reading matter on both days. The type and size of the letters was therefore the same during the two recordings. The subjects brought their own reading matter in order to ensure concentration and cooperation. Because of the concentration factor, the recording time was also kept short: 2 x 45 min with a 15 min coffee break. After the first 45 rnin of recording their reading posture, the subjects were asked to write for Smin in order to get an impression of the influence of an inclined desk on writing posture. Before, between and after the two sessions, a gauge was recorded on tape. This involved recording the 'zero position* in addition to some extreme body positions. The zero position is the position in which the subject stands erect with an upright head, and with the left thumb in the right fist. A mark on the wall at 5 m distance ensures that the subjects looks forward at an angle of 15" below the horizontal. Besides the four inclinometers, use was made of an instrumentation recorder (Racall Store 14). For a detailed description of the inclinometer, see'snijders et a L (1987). An inclinometer measures its angle in relation to the direction of gravity. The aforementioned zero position is given the value 0".A forward bending posture gets a positive angle, and a backward bending, a negative angle. This method has the advantage that if the inclinometer was attached at a slightly different place during the second recording, then it would not influence the results. It also reduced individual differences. The analogue signal (figure 3) was digitized and fed into a computer (PDP 11/73). During this procedure, the tape was accelerated to 32 times the recording speed. In order to record spinal movements with a frequency up to 3 Hz, a filter frequency up to 96 Hz and a sample frequency of 480 Hz were used. With the aid of a number of computer programs, each signal was converted into a histogram in which, using 2" intervals, the time that the person stayed in a posture is given as a percentage (Snijders et al. 1987). By computing the median of the aforementioned percentages an indication can be obtained of the subject's preferred posture. The range of posture lies between the position of the 5th and 95th percentiles in the histograms (so posture is in this range 90% of the time). 4. Statistics Whether the use of an inclined desk as opposed to a flat desk result in a change in preferred posture can be determined by comparing the aforementioned medians of the two recordings of the subjects. A difference in sitting behaviour can be identified by comparing the ranges of posture in these recordings.
579
Downloaded by [University of Arizona] at 00:04 08 September 2015
The efSect on posture of an inclined desk
looJ scc o
. . . . . . . . . . . . . . . . . . . . . . . . . r
too
.
r
is0
1
.
1
.
,
1
200
.
1
.
is0
Figure 3. The analogue signal of two inclinometers.
Wilcoxon's test was used to compare quantitative differences between paired observations against the zero hypothesis, because the elements of the random test come from a symmetrically distributed population with a mean of zero. 5. Results An example of the calculated histograms for one person are presented in figure 4. This figure shows the histograms of the sagittal flexion of the head, trunk and the angle between head and trunk during one session with the flat desk. The histograms of most of the subjects show a shift to a more upright posture when using the inclined desk. An example is given in figure 5, in which working at the inclined desk is indicated by the shaded histogram and working at the flat desk by the black histogram. The medians of these histograms were taken to be the preferred postures. In the reading test, the mean value of the preferred postures of the trunks of the 10 subjects
-
Downloaded by [University of Arizona] at 00:04 08 September 2015
M.de Wall et al.
Figure 4. Result of the canversion of the analogue signal into histograms. Zero degrees corresponds with a posture in the 'zero position'.
in the sagittal plane was 7"more upright. One subject had a 28"more upright position of the trunk. For the head, the change in preferred posture was an elevational of 6" to a more upright position (with a maximum of 16' for two subjects). Both differences proved to be significant (pt0.05)(table 1). The angle between the head and trunk in the sagittal plane did not change significantly. Similarly, the inclined desk did not result in a difference in the preferred posture in the frontal plane (table 1).
.
Downloaded by [University of Arizona] at 00:04 08 September 2015
The eflect on posture ofan inclined desk
0 -20-10
0 10 20 X) 40 w l e between head ud mrk
50
60
70
Iawrnl
Figure 5. Examples of histograms of one subject when reading with an without an inclined desk.
As we11 as decreasing during the reading test, the angEe of the trunk in the sagittal plane also decreased by 9" when the subjects wrote at the inclined desk (table 3). However, a decrease in the sagittal angle of the head during writing could not be demonstrated. The range of trunk postures in the frontal plane decreased by 4" (pt0.05)when
M. de Wall et al.
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Table 1. Mean of the preferred postures of the 10 subjects in degrees in relation to the 'zeroposition' during the reading test. Inclined desk
Posture
32 23 9 4 2 3
Head in sagittal plane
Trunk in sagittaI plane Head-trunk in sagittal plane Head in frontal plane Trunk in frontal plane Head-trunk in frontal plane
Flat desk
38 ( 2 7 . 1 ) 30 ( i 7 . 5 ) 8 (k6.0) 6 (26.7) 1 (k2.8) 5 (k5.3)
(k6-9) (k4.5) (rt 7.0) (*3-9) (~2.2) ( k 5.1)
Difference - 6 (k5-7)* -7 (k8.6). 1 (k7.4)
-2 (k3.9) 1 (k2.7) -2 (-3.0)
Downloaded by [University of Arizona] at 00:04 08 September 2015
*pcO.05; Wilcoxon's test.
Table 2. Mean of the ranges of posture (the difference between the 5th and the 95th percentile) of the 10 subjects in degrees during the reading test. Inclined -
-
Flat desk
desk
Posture - -
Head in sagittal plane Trunk in sagittal plane Head-trunk in sagittal plane Head in frontal plane Trunk in frontal plane Head-trunk in frontal plane
-
24 27 30 24 :
Difference -
(rt 8.5)
24 ( k 8.5) 20 ( * 4.1) 26 ( * 7-5) 27 (k 10.0) 1 6 ( * 5-9) 27 (k 12-9)
( 2 10-4)
(k10.6) (*10.6)
12 ( 2 4-0) 23 ( k .. 8.8)
0 ( * 7-4) 7 (i17-1) 4 ( A 13.3) -3 (+. 4-9) -4(* 2.3). -4 (-c 8-2)
*pt0.05; Wilcoxon's test.
reading at an inclined desk (table 2). This could not be demonstrated during the writing test, perhaps because of its short duration (5 min). 6. Discussion This method has been a workable one for recording subjects' posture. Technical defects did not occur and the test had good reproducibility. The value of the 'zere position' was measured three times at each session. The three 'zero positions' of each '
Table 3. Mean of the preferred postures of the 10 subjects in degrees in relation to the 'zem position' during the writing test. Posture
Head in sagittal plane
Trunk in sagittal plane Head-trunk in sagittal plane Head in frontal plane Trunk in frontal plane Head-trunk in frontal plane
*p
